1. Field of the Invention
The present invention relates to a valve timing control system for an internal combustion engine, for controlling valve timing of intake valves of the engine, and a control system for an internal combustion engine, for controlling boost pressure of intake air via a supercharging device and variably controlling valve-closing timing of intake valves with respect to valve-opening timing thereof as desired via a variable valve timing device.
2. Description of the Related Art
Conventionally, a valve timing control system for an internal combustion engine, for controlling valve timing of intake valves of the engine has been proposed e.g. in Japanese Laid-Open Utility Model Registration Publication (Kokoku) No. S56-9045 (pages 1 and 2, FIGS. 1 to 4). This engine includes a rocker arm for opening and closing each intake valve, and two cams, one for low load (low-load cam) and the other for high load (high-load cam), provided for the intake valve, and an actuator for switching the cam for actuating the rocker arm between the low-load cam and the high-load cam. The two cams are arranged adjacent to each other on a cam shaft, and have respective cam profiles configured such that the cam profile of the low-load cam provides the same valve-closing timing but more retarded valve-closing timing as compared with the cam profile of the high-load cam. Further, the rocker arm is provided on the rocker shaft in an axially slidable manner, and is actuated by the actuator such that it is slid between a position in which one end thereof is in abutment with the low-load cam and a position in which the same is in abutment with the high-load cam. In short, the actuator switches the cam for actuating the intake valve between the low-load cam and the high-load cam.
In this valve timing control system, through the control of the actuator, the cam for actuating the intake valve is switched to the low-load cam when the engine in a low-load region, and to the high-load cam when the same is in a high-load region, whereby the valve-closing timing of the intake valve is switched such that it is retarded when the engine is in the low-load region than when it is in the high-load region. The switching of the valve-closing timing is performed for the following reason: The amount of air to be drawn into the cylinder is smaller in the low-load region of the engine than in the high-load region of the same, and if the amount of intake air is reduced by reducing the degree of opening of a throttle valve, pumping loss occurs due to the lowering of pressure (increase in negative pressure) within the intake valve, resulting in degraded fuel economy. To avoid this inconvenience, the valve timing control system described above causes blow-back of intake air from within the cylinder by retarding the valve-closing timing of the intake valve in the low-load region than in the high-load region, whereby the amount of intake air drawn into the cylinder is reduced without reducing the degree of opening of the throttle valve.
Also, a valve timing control system for an internal combustion engine, for controlling valve timing of intake valves of the engine has been proposed in Japanese Laid-Open Patent Publication (Kokai) No. H07-269380 (pages 2 and 3, FIG. 7). This valve timing control system controls the phase of an intake cam for actuating each intake valve is controlled such that it is varied with respect to that of the crankshaft of the engine, whereby the intake valve has its valve-opening and valve-closing timing varied while holding a constant valve open time period.
According to the valve timing control system proposed in Japanese Laid-Open Utility Model Registration Publication (Kokoku) No. S56-9045, the cam for actuating the intake valve is merely switched between the low-load cam and the high-load cam, and hence, the valve-closing timing of the intake valve can only be switched between two types of timing determined by the respective cam profiles of the two cams. Therefore, this system is incapable of controlling the amount of intake air in an accurate and fine-grained manner, so that the amount of intake air may become short or excessive to make combustion unstable, degrade fuel economy, and increase exhaust emissions. If the valve timing of each intake valve is controlled by the valve timing control system proposed in Japanese Laid-Open Patent Publication (Kokai) No. H07-269380 in order to control the amount of intake air, the valve-opening and valve-closing timing of the intake valve are changed with the valve open time period of the intake held constant. This brings about a change in the valve overlap, which makes it impossible to provide control on the amount of intake air in a fine-grained manner, and at the same time degrades combustion performance of the engine and increase exhaust emissions due to changes in internal EGR.
Further, a control system for an internal combustion engine, for controlling boost pressure of intake air via a supercharging device has been conventionally proposed in Japanese Patent Publication (Kokoku) No. H03-37034 (pages 3 to 5, FIG. 3). This engine includes a turbocharger as the supercharging device and a knock sensor. In this turbocharger, as the degree of opening of a wastegate valve changes, the flow rate of exhaust gases to turbine blades is changed, whereby the boost pressure is changed. Further, the knock sensor is for detecting knocking in the engine, and delivers a pulse signal indicative of occurrence of knocking to the control system.
The control system counts the number of occurrences of knocking during one combustion cycle of the engine based on the pulse signal from the knock sensor, and sets a retard value for retarding ignition timing according to the number of occurrences of knocking during one combustion cycle and also a duty ratio of a drive signal to the wastegate valve according to the number of occurrences of knocking during four combustion cycles. More specifically, the retard value of ignition timing is set to a larger value, i.e. such that ignition timing is more retarded, as the number of occurrences of knocking during one combustion cycle is larger. Further, the duty ratio of the drive signal to the wastegate valve is set to a smaller value, i.e. such that the boost pressure becomes lower, as the number of occurrences of knocking during four combustion cycles is larger. Thus, when knocking occurs, the ignition timing is retarded and the boost pressure is lowered, whereby occurrence of knocking is suppressed.
According to the control system proposed in Japanese Patent Publication (Kokoku) No. H03-37034, when knocking occurs, ignition timing is retarded and at the same time boost pressure is lowered. This makes it possible to suppress occurrence of knocking in the high-load region of the engine, but due to execution of such control, combustion efficiency and engine output are both lowered, resulting in degraded drivability and lowered marketability.